 In today's video, we will try and identify the effect of a nucleophile on the rate of an SN2 reaction. But before we proceed, why don't we just recall the mechanism of an SN2 reaction? In this reaction, the attacking nucleophile Y- attacks the carbon that is attached to the leaving group X and X- leads. All this happens in the same step. So the stronger the attacking nucleophile, the more easily it would attack, the faster it would attack and the more would be the rate of the reaction. So yes, one of the factors that affects the rate of an SN2 reaction is a nucleophile. So why don't we compare a few nucleophiles to understand this concept better? Which of the following nucleophiles is the strongest? And we have the options as N2-, OH-, F- and Cl-. Well, if the nucleophile or if this negatively charged ion is stable, why would it want to attack? Right? So a strong nucleophile would be the one that isn't really stable alone. It really wants to attack that electrophilic. So we can say that the reactivity or the strength of a nucleophile is inversely proportional to its stability. Great! So all we have to do is compare the stabilities of these four ions and the one that's the least stable would be the strongest nucleophile or we can say it would be the most reactive nucleophile, right? And how do we compare the stability of anions? Right! Electronic effects. We start by looking at the atom on which the negative charges. Then we compare the sizes, then the electronegativities and if we are not coming on to a result using these, what comes handy? The inductive effect, the resonance, etc., etc. So let's begin. Hey chlorine is the largest, right? And since the chlorine atom is the largest, the chloride ion is the largest, so the charge is spread out more and the charge per unit area is less, the chloride ion is the most stable. What about the rest? Well, all three belong to the same period. So there's not much difference in their size. But we can think of electronegativity. There is more electronegative than oxygen and oxygen is more electronegative than nitrogen. The one that's the most electronegative one would have the highest tendency to keep the shared pair of electrons towards itself or also to keep the electron density towards itself. So one that's most electronegative would keep the electron density or the negative charge in a much stable manner than the other two, correct? So F minus would be more stable than OH minus which would be more stable than NH2 minus. So what would the stability look like? 4 is more stable than 3 which is more stable than 2 which is more stable than 1 and therefore what would the reactivity look like? Well, exactly opposite to this, correct? So the strongest nucleophile would be NH2 minus. Let's go to the next problem now. Identify the stronger nucleophile, CH3O minus and PHO minus. Stability or the strength of a nucleophile is inversely proportional to stability. Here in either case the negative charge is present on the oxygen atom. Okay, what do I look at now? Well, the attached species. In the first case it is attached to CH3 while in the second case this oxygen atom is attached to the benzene drink. Which one would be more stable? Well, the CH3 group is an electron donating group. It would push electron density towards this oxygen atom via sigma bonds. It would be depicting a plus i effect. The oxygen atom already has a negative charge and somebody is pushing more electron density on it. It would be unstable, right? It wants someone to take away this electron density and that is exactly what the benzene drink does. The benzene drink withdraws electron density via resonance and this negative charge would delocalize throughout the entire ring, correct? So which one would be more stable? The second one would be more stable than the first one and therefore the stronger nucleophile would be the first one. Now this is the usual way of doing it. What about when they specify the solvent? Does the solvent play a vital role in deciding the strength of the nucleophile? Let's find out. Well, here we will be comparing the strength of the following nucleophiles in different solvents. On the left hand side we will compare their strengths in water while on the right hand side we will compare their strengths in DMSO. Well, water is a polar protic solvent while DMSO or dimethyl sulfoxide is a polar A-protic solvent. There is no edge here. The electrophilic center is not hydrogen, it's sulphur here. So what's the difference in these cases? Well, do you folks remember anything about solvation or hydration? Whenever there is an iron in water, the water molecules try and hydrate it. There is an iron dipole interaction and this iron gets hydrated. The partially positive hydrogen from the water molecules surround the negatively charged nucleophile. So which one would be more hydrated? The stronger that negative charge is, the more it would attract these polar molecules towards itself. But here all three have the same amount of charge that is minus one, correct? But their sizes are different. So the smaller the size of the iron, the more concentrated the charge would be and the more concentrated the charge would be, the more it would attract the water molecules towards itself. So which of the three would be hydrated the most? It would be the fluoride iron and the more hydrated it is, the more water molecules would be around it and its effective size would increase. So if we are asked which of the following would have the largest effective size in water? So let's say their original sizes look something like this. Once they get hydrated in water, their effective sizes would look something like this. Since fluoride iron is the most hydrated one, it would have the largest effective size and now this large size thing has to go and attach itself to an electrophilic sector. Do you remember how an SN2 reaction is a single step reaction? This large sized iron has to come and attack it. Wouldn't it be facing a lot of steric hindrance? So wouldn't it be a good nucleophile? No, right? So in water which would be the best nucleophile? It would be bromide iron. But what happens if we take these ions in DMSO? Well, the electrophilic center in DMSO is pretty hidden, isn't it? There's this and then there's these big groups attached. It is not really oriented well enough to try and surround these negatively charged ions well. So there would be no solvation in this case. Fluoride iron would have the smallest size, it would have the most concentrated charge and therefore it would be the strongest nucleophile out of three.